This invention relates to a calibration system for a magnetic heading sensor, or compass, in a vehicle, and, more particularly to a continuous and fully automatic calibration system for the compass in the vehicle. The invention can be applied to a magneto-inductive sensor, magneto-resistive sensor, flux gate sensor and other known sensing technology.
Many vehicles today are equipped with magnetic compasses to determine the direction in which the vehicle is heading and convey such information to the passengers and/or driver of the particular vehicle. Generally, these magnetic compasses include a magneto-responsive sensor, such as a magnetic rotor sensor, a flux gate sensor, a magneto-resistive sensor, a magneto-inductive sensor, a magneto-capacitive sensor, or the like. The magneto-responsive sensor detects the magnetic field present in the vicinity of the vehicle and processes this signal in order to determine a directional heading of the vehicle relative to the Earth magnetic field. However, these magnetic compasses must be calibrated within the vehicle to account for any deviating magnetic field from the vehicle or other structures surrounding the vehicle, in order to determine the true heading of the vehicle relative to the Earth magnetic field. Additionally, as the deviating field may change over time, due to either a change in the magnetic signature of the vehicle or in the magnetic field surrounding the vehicle, an original calibration may later become less accurate.
To date, there have been compasses proposed which include automatic magnetic compass calibration within a vehicle after an initial manual adjustment or preset has been made to the compass system. Such calibration processes are performed as the vehicle is driven in order to account for changes in the deviating magnetic fields, thereby maintaining an accurate directional readout to the driver of the vehicle. However, many of these calibration systems require the vehicle to be oriented in specific directions relative to Earth magnetic field, such as orienting the vehicle in 180 degree opposite directions or driving the vehicle in a complete 360 degree circle while data is sampled by the system, in order to calibrate the system. Many other systems alternatively require complicated mathematical functions to determine the true Earth magnetic field based on several data points collected as the vehicle is driven in several directions. While these systems may provide an accurate calibration in many areas of the Earth, they are often based upon an assumption that a trace or plot of the Earth magnetic field is substantially circular in shape, as plotted on a Cartesian coordinate system relative to the vehicle. In reality, however, irregularities may occur in the mounting of the compass and/or sensors in the vehicle such that the sensors may be tilted relative to the magnetic field of the Earth. Furthermore, the variations or declinations present in the Earth magnetic field at any given location are generally not perfectly symmetrical, as the declination varies irregularly over the Earth surface and further varies over a period of time. These irregularities and variations may result in a substantially non-circular or oval-shaped trace of the magnetic field rather than the circular field that many of the proposed calibration systems are based on.
An additional concern with the systems proposed to date is that the initial deviating magnetic field or magnetic signature of the vehicle must be offset so that the magneto-responsive sensor""s output will be within an operable range of the electronic processing system. As a vehicle is manufactured, or shortly thereafter, the deviating field of the vehicle may be substantially offset or nullified by an initial preset of the vehicle""s magnetic signature, which brings an origin of the Earth magnetic field to within a predetermined range of a center or origin of the compass coordinate system. Once the vehicle has its magnetic signature preset, the algorithmic or digital calibration systems may be implemented to refine the vehicle compass to within a desired range of accuracy.
Furthermore, the magnetic signature of a vehicle may change significantly over time, such as when a new sunroof motor is installed in the vehicle, a magnetic antenna is mounted to the vehicle or the like. If the magnetic signature changes too much, the Earth magnetic field, as sensed by the sensors, may be shifted out of the operable range of the analog-to-digital converter of the calibration system. In order to re-set the deviating magnetic field of the vehicle such that the sensed Earth field is back within the window of the calibration systems, the compass system may again need to be manually adjusted by a mechanic or technician.
Therefore, there is a need in the art for a fully automatic and continuous calibration system for calibrating a magnetic compass located on a vehicle. The calibration system must be able to account for the deviating magnetic field of the vehicle without requiring an initial preset or demagnetization of the vehicle as the vehicle is assembled. Furthermore, the calibration system must continuously account for minor and major changes in the deviating magnetic field by digitally or physically adjusting for such changes. These adjustments must also account for both circular and non-circular Earth magnetic fields. Furthermore, the calibration system must account for major changes in the deviating magnetic field in order to avoid requiring manual adjustments throughout the life of the vehicle.
The present invention is intended to provide a fully automatic and continuous calibration system for vehicle compasses, which continuously calibrates the compass without requiring an initial preset of the vehicle magnetic signature or manual calibration of the compass.
According to an aspect of the present invention, a vehicular electronic compass system comprises a magneto-responsive sensor for sensing a magnetic field and an electronic circuit responsive to the sensor assembly for determining either the magnitude or direction, or both, of the Earth magnetic field. The sensor detects at least one data point and the electronic circuit adjusts the sensor according to an approximation of a center of the Earth magnetic field calculated from the at least one data point and an estimated value of Earth magnetic field magnitude. The system further includes a display coupled with the electronic circuit for displaying a direction of the Earth magnetic field.
According to another aspect of the invention, a vehicular electronic compass system includes a magneto-responsive sensor assembly for sensing a magnetic field and an electronic circuit responsive to the sensor assembly for determining either the magnitude or direction, or both, of the Earth magnetic field. The electronic circuit collects a plurality of data points relative to a coordinate system associated with the vehicle. Each of the data points has at least two components relative to the coordinate system. The electronic circuit averages at least one component of at least two of the data points to determine an estimated value of the Earth magnetic field along at least one axis of the coordinate system. In this manner, an estimated offset of the Earth magnetic field is calculated and the electronic circuit adjusts a directional heading output to account for the offset. An electronic display is coupled with the circuit for displaying a direction of the Earth magnetic field.
According to yet another aspect of the invention, a vehicular electronic compass system includes a magneto-responsive sensor assembly for sensing a magnetic field and an electronic circuit responsive to the sensor assembly for determining magnitude, direction, or both, of the Earth magnetic field. The electronic circuit collects data points, determines from the collected data points an offset value of the Earth magnetic field and adjusts a directional heading output of the sensor assembly to account for the offset value. The electronic circuit includes an extended range calibration function for identifying a change in magnetic signature from at least one of the sensor assembly and the offset value. The electronic circuit adjusts the sensor assembly in response to a change in magnetic signature. The system further includes an electronic display coupled with the electronic circuit for displaying a direction of the Earth magnetic field.
According to a more detailed aspect of the invention, a vehicular electronic compass system includes a magneto-responsive sensor assembly for sensing a magnetic field and a microcomputer system responsive to the sensor assembly for determining magnitude, direction, or both, of the Earth magnetic field. The microcomputer system collects data points, determines from the collected data points an offset value of the Earth magnetic field and adjusts a directional heading output of the sensor assembly to account for the offset. The microcomputer system occasionally calculates a new value of the offset. The microcomputer system has a digital-to-analog converter converting digital values to analog signals for adjusting the sensor assembly and an analog-to-digital converter having a range of operation for converting outputs of the sensor assembly to digital values. The microcomputer system adjusts the sensor assembly and calculates a new value of offset in response to either i) an output of the sensor assembly exceeding the range of operation of the analog-to-digital converter, ii) an abnormal relationship between collected data points and the value of the offset, or iii) a change in value of the offset which exceeds a predetermined amount. The system further includes an electronic display coupled with the microcomputer system for displaying a direction of the Earth magnetic field.
A calibration method for calibrating a compass for use on a vehicle according to an aspect of the invention includes sampling at least one data point of a magnetic field with a magnetic sensor, determining coordinates for the at least one data point relative to an origin of a coordinate system associated with the vehicle, estimating an origin of the Earth magnetic field from an estimated value of the Earth magnetic field magnitude and at least one data point and adjusting the sensor to offset the estimated origin to the coordinate system associated with the vehicle.
According to another aspect of the invention, a calibration method for calibrating a compass for use on a vehicle includes sampling at least one pair of data points that are substantially oppositely positioned relative to an axis of a coordinate system, averaging the substantially opposite values of the pair of data points to determine a deviation from a zero value of the coordinate system and adjusting an output of the system as a function of the deviation.
According to yet another aspect of the invention, a calibration method for calibrating a compass for use on a vehicle includes collecting data points with a sensor assembly, determining from the collected data points an offset value of the Earth magnetic field and adjusting a directional heading output of the sensor assembly to account for the offset. The method further includes identifying a change in magnetic signature from at least one of the sensor assembly and the offset value and adjusting the sensor assembly in response to a change in magnetic signature.
These and other objects, advantages, purposes and features of this invention will become apparent upon review of the following specification in conjunction with the drawings.